I've seen a few theories in various places: - Match the type of accidentals used by the key signature. - Use sharps when the melody is moving up, and flats wh

I've seen a few theories in various places: - Match the type of accidentals used by the key signature. - Use sharps when the melody is moving up, and flats when it's moving down. - Use whatever accidentals give you a visually pleasing spacing of notes in "thirds" as represented on the staff. (As advocated here.) - Use the accidentals that are physically compatible with other notes in the chord. (Only a concern for certain instruments, but e.g. I understand that the pedals on a harp affect many strings at once, so playing a particular combination of notes may require a clever approach to the pedals, indicated by what might otherwise look like an unnatural set of accidentals. This wouldn't apply to a piano, where G# is just a separate key from G, not a modification of the G key.) I think this is what you're going for?